The ISO (International Organization for Standardization) has a standardized framework (14040) for LCA, which divides the entire LCA procedure into four distinct phases: goal and scope definition, inventory analysis, impact assessment, and interpretation ( ISO, 2006 ). A full LCA includes the four stages ( Guinée, 2002 ). An LCA that is limited to addressing the contribution to climate change is usually called a CF (carbon footprint) or climate change impact assessment ( Cederberg et al. , 2013 ). A clear system boundary is essential to include all the emissions caused by inputs that contribute to production ( Suh et al. , 2004 ). A cradle to farm gate system was adopted for the study and harvest of barley was defined as the ending point of the system. Indeed, the focus was on the production at the farm level during farming season rather than storage, distribution and consumption.

Ten environmental impact categories were considered in this study including: abiotic depletion potential (AD); acidification potential (AC); eutrophication potential (EU); freshwater aquatic ecotoxicity potential (FAET);global warming potential for time horizon 100 years (GWP); ozone depletion (OD) potential; human toxicity (HT) potential; marine aquatic ecotoxicity potential (MAET); photochemical oxidation potential (PhO); and terrestrial ecotoxicity (TE) potential. For the data analysis, CML 2 baseline 2000 V2/world developed by the Institute of Environmental Science of Leiden University ( PRéConsultants, 2003 ) in SimaPro 8 was applied.

Two main LCA models were provided for each crop; i.e . irrigated and rain-fed barley. A sub-model was designed for farm machinery operations including: (a) for irrigated barley: ploughing, chiseling, planting, fertilizing by broadcaster, spraying by field sprayer and harvesting; (b) for rain-fed barley: chiseling, sowing by broadcaster, fertilizing by broadcaster, spraying by field sprayer and harvesting.

The above model was then entered into the main LCA model involving all inputs ( e.g . fertilizers, pesticides, etc.), barley seed as output and different emissions into the air and water. Transportation of inputs was also considered as ton per kilometer (functional unit FU = tkm) by tractor and trailer. Four additional scenarios were considered for irrigated barley, and suitable LCA models were accordingly designed to analyze how alternative energy use options affected the environmental impacts of this crop. The scenarios were: S1, irrigated barley production as reference scenario; S2, using no-tillage machinery to convert ploughing, chiseling and planting to one operation; S3, using biomass for electricity generation versus gas utilization (as a fossil fuel) at power plants; S4, combination of S2 and S3 to analyze whether environmental impacts can be mitigated by reduction of tillage intensity and gas consumption at electricity power plants. These scenarios were compared to irrigated barley LCA model (S1) as reference scenario.

Three different functional units (FUs) have been proposed for LCA studies ( Nemecek et al ., 2011 ): 1) the land management function, measuring as cultivated hectare per year (land-based); 2) the financial function expressed as a currency unit; and 3) the productive function described by physical units ( e.g ., kg dry matter yield, MJ net energy for lactation-mass-based). The land-based FU was used in the model for farm machinery operations to calculate environmental impacts per hectare. Then the mass-based FU was employed in the main LCA model to analyze the result based on kg dry barley yield. Application of two or more FUs can better clarify environmental performance ( Van der Werf et al ., 2007 ).

Emissions

The IPCC (2006) guidelines were used to determine nitrous oxide (N ₂ O) emission into the air. Accordingly, the application of 100 kg of N-based fertilizers emits 1.25 kg of N ₂ O into the air. It was assumed that 30% of total N fertilizers leached into groundwater in the form of nitrate ( Erickson et al ., 2001 ).

The average amount of P leached to groundwater was estimated as 0.07 kg/ha of P-based fertilizers ( Nemecek et al ., 2007 ). It has been suggested that 30-50% of total sprayed pesticides be considered as emissions into the air ( Sahle & Potting, 2013 ).

The amount of diesel fuel used for farm operations and pumping water and amount of electricity for pumping water were estimated and equivalent CO ₂ emission was considered in the model.

Table 2 indicates that the average farmers’ ages are not significantly different in IFs and RFs (39.5 and 38.2 yrs old, respectively). Nonetheless, the farming experiences were higher than 2 yr on average in IFs (15.5 vs . 13.5 yrs). Another study in the West of Iran by Zarafshani et al . (2017) indicated that canola farmers have higher average age (44.18 yrs) than our barley farmers. Almost all (99.7%) of the barley growing farmers were male. The average size of farming was significantly different between the studied groups (3.4 ha for IFs and 5.8 ha for RFs). However, the net incomes were also significantly different due mainly to different yields in IFs and RFs. The average net income was 510.5 $/ha in IFs and 250.3 $/ha in RFs. Several studies confirmed that farmers’ age, experience and net income influenced the agricultural practices enhancement and adoption of new technologies ( Adesoji & Farinde, 2006 ; Dinpanah & Naji, 2012 ; Shojaei et al ., 2013 ).

The calculated values for energy consumption in the barley fields are given in Table 3 . Production of barley in IFs consumed ~ 25,000 MJ/ha, which was twice the energy of rain-fed barley growing systems; the yield of IFs was also twice that of the RFs. Statistical analysis revealed that irrigated and rain-fed barley had significantly different energy input and outputs at a 0.01 level of significance.

TTable 3 shows that all energy efficiency indices in IFs were higher than in RFs. Output-input energy ratios were close, 1.33 in IFs and 1.25 in RFs. These values were half of that found by Finnish farmers ( Mikkola et al. , 2011 ) and one fifth of Australian farmers ( Khan et al ., 2010 ). However, net energy gain in IFs was around 8300 MJ/ha, 2.5 times that of RFs. Specific energy consumption for barley cultivation was 3.5-4.5 MJ/kg in Italy ( Pellizzi (1992) and 1.62 MJ/kg in Spain ( Lechon et al ., 2005 ), while in our Iranian barley farms it was much higher (11 MJ/kg). Barley grain yield in IFs was similar to that found by Fallahpour et al. (2012) in Khorasan, Northeast Iran, with a similar N consumption (120 kg/ha), whereas the yield of RFs was around twice of Khorasan farms (60 kg/ha N consumption). This shows that our farmers can reach higher yield with almost similar N application.

Fertilizer, diesel fuel and water for irrigation were the main contributors of energy input in IFs (~ 78%). The share of fertilizer and diesel fuel were also dominant in RFs. Both barley production systems consumed high amount of N with the share of 90% of fertilizer energy factor. Our farmers use around three times more pesticides compared to Finnish farmers.

An experimental study determined in Agramunt (NE Spain) that application of 40 kg N lead to 4.5 and 6.3 ton/ha of barley yield under rain-fed and irrigated conditions, respectively ( Cossani et al ., 2009 ). This shows that our farmers apply much more N in their farms. Compared to Finnish farmers ( Mikkola et al ., 2011 ), in this work irrigated barley producers consumed ~ 50 kg/ha more N and ~ 20 kg/ha more K. Integrated methods to increase soil fertility can be a useful choice since it has been shown that application of biochar and N fertilizer on a light soil within temperate climate increased by 30% the yield of spring barley compared to N fertilizer only ( Gathorne-Hardy et al ., 2009 ).

Although some farmers use legal amounts of this input, a large number of them do not usually conduct soil sampling to understand how much and what type of fertilizer should be applied. No green crops such as alfalfa and clover are currently planted in barley farms. The intensive cropping of barley, wheat and corn, especially at IFs farms, has led to continuous soil infertility. The unnecessary use and waste of N is critical since it is very mobile in the soil and embedded high energy and, consequently, it may reduce the sustainability of barley production. The mobility of N can be dangerous in both IFs and RFs; in the IFs, due mainly to leaching into underground water resources and in the RFs due to emissions into the air ( Khan et al ., 2009 ; Meisterling et al ., 2009 ). At global scale, fertilizer-induced N ₂ O emissions and nitrate leaching have been estimated to be approximately 0.8% and 19% of N fertilizer input, respectively ( Kim et al ., 2015 ). Using green crops and crop rotation would be useful to maintain soil fertility and to reduce chemical N consumption. Eriksen et al. (2015) reported that rotation of barley and ryegrass led to lower nitrate leaching compared to other experiments with barley-pea and old grasslands rotations.

Although less farm operations have been done in the RFs, the share of diesel fuel in energy input was higher than in the IFs (33% vs 22%; T Table 3 ). One of the deficiencies in RFs is probably related to the machines used for planting. Technically, the planter should place the seeds into the soil deeper under rain-fed than under irrigated conditions and particular press wheels provides suitable contact between the soil and seeds. In more than 95% of the RFs, seeds are broadcast on the soil surface and then a shallow disking is used to cover seeds. In such situations, germination is not satisfactory to approach higher yield. Near to 85% of available tractors and equipment exceeded the economic and technical life spam and should be replaced by new machines. Using new machines would be useful to increase energy input efficiency. Our observations showed that RFs need more suitable machinery in the region. Additionally, it is recommended that conservation tillage systems such as minimum or no-tillage should be examined locally in the province and be introduced to the farmers.

Energy consumption in the forms of water for irrigation and electricity for pumping water alone accounted for around 23% of total energy input in IFs. It is a critical point since Iran is located in a semi-arid region encountering water shortage. One reason is that most farmers use furrow irrigation systems while more efficient irrigation systems like sprinkler may improve water use efficiency and productivity; i.e . less water consumption and higher yield. Water productivity in our IFs was much lower (0.5 kg barley per cubic meter of water) than in Australia (~ 3.5 kg/m ³ , Khan et al ., 2010 ) or Portugal (1.8 kg/m ³ , Paredes et al. , 2017 ). Water productivity was also high in China for grain crops (~ 1.58-1.72 kg/m ³ ) by employing novel irrigation methods ( Kang et al ., 2016 ). It should be noted that only water from wells were considered in the analysis of IFs to reach correct assessment of energy efficiency and productivity. In the study by Paredes et al . (2017) , both irrigation and rainfall water were included. Thus, differences in water productivity between our farms and the farms in Portugal may be due to different basic assumptions made.

Environmental impacts of farm machinery operations

As described before, a separate LCA model was provided for machinery operations to analyze each operation in more detail. Fig. 1 shows that in IFs, chiseling, plowing, planting and combine harvesting had almost similar impacts in all studied LCA categories; i.e. AD, AC, EU, GW, OD, HT, FAET and MAET, TE, PhO by around 10% for chiseling, 20% for plowing and planting and 32% for harvesting. Likewise, the impact of chiseling was around 20% and combine harvesting around 50% in RFs ( Fig. 1 ). Emitted heavy metals from direct energy consumption in machines usually contributed to ecological toxicity. Some parts of acidification were also caused by air emissions of SO ₂ , N ₂ O and ammonia.

Diesel fuel consumption in farm machines had the highest contributions to AD and OD in both irrigated and rain-fed conditions (~ 45-50%). This is critical since the share of diesel fuel in energy input was also significant (~ 20-30%). Diesel as a source of fossil fuel affected abiotic directly and affected OD by emitting greenhouse gases. Accordingly, the given suggestions to reduce diesel fuel input are emphasized here to mitigate burdens on the environment. Another essential key is that efficient tractors should be applied in farms to reduce diesel inputs. Lares-Orozco et al . (2016) , using more efficient tractors that decrease diesel inputs, found emissions reduced a major 33% under conventional tillage, and a 24% under no tillage.

Environmental impacts of barley production

The results of LCA for irrigated and rain-fed barley are shown in Fig. 2 . Electricity consumption in IFs was responsible for the highest environmental burdens in several impact categories including AD, GW, HT, FAET, MAET and PhO. A direct comparison among the results from different LCA studies is not always straightforward due to the different system boundaries definition and assumptions. Niero et al . (2015) reported different results on barley production in Denmark in which environmental impacts of fertilizers, especially N, dominated, Danish farmers consumed around 240 kg of different N fertilizers vs 140 kg in Iran. Rajaniemi et al . (2011) reported that Finnish farmers applied ~ 86 kg/ha N to obtain 3380 kg/ha of barley. However, climate conditions, soil type and other variables can have an effect on the different N consumptions.

Abiotic resources are considered as natural resources and energy generation usually affected these resources. Accordingly, abiotic depletion is strongly dependent on electricity generation and consumption. Given the findings of the study, using efficient electrical water pumps at the farm level is essential to mitigate electricity energy consumption and, consequently, environmental impacts. Electricity generation from agricultural waste and biomass can be recommended as an alternative way to reduce the environmental effects of electricity especially on the AD. In IFs, the impact of farm operations on the OD (~ 30%) was similar to that of electricity. However farm operations, compared to electricity and irrigating, had much less impact on AD, AC, EU, HT, FAET, MAET, TE and PhO. Irrigating had similar environmental impacts regarding AD, AC, EU, GW, OD and HT potential. Instead, TE is mainly due to contribution of irrigating life cycle (~ 80%). Impacts of farm machinery operations in RFs were dominant in almost all impact categories ( Fig. 2 ). It seems that farm operations had highest environmental impacts in RFs since no electricity and water from wells were used in these farms compared to IFs.

Urea as an energy intensive input mostly affected terrestrial ecosystem by 40% and abiotic by 28% in RFs. However, the effect of urea on the MAET, OD, HT and FAET was also considerable (~ 20%). Brentrup et al. (2004) revealed that increasing N fertilizer rates, which do not result in an equivalent yield increase per ha, lead to increasing CO ₂ rates per ton of grain. Accordingly, it is essential to know how much fertilizers are needed for each kilogram of barley in each special farm/region to reduce these environmental impacts.

Impacts of potassium sulphate, ammonium nitrate and agricultural machinery on the FAET, MAET, TE and PhO were similar, achieving values of 10-18%. Meanwhile, transportation and glyphosate consumption had negligible burdens on the environment in all the considered categories. Comparison of environmental impacts of IFs and RFs revealed that IFs burden more the environment than RFs in all impact categories ( Table 4 ). Considering global warming potential, for instance, the emitted kg CO ₂ eq. from IFs was around twice of RFs; i.e . 2.35 vs . 1.31 kg CO ₂ eq. ( Table 4 ). This result is consistent with another study in Khorasan, Northeast Iran ( Fallahpour et al ., 2012 ) which indicated that IFs have higher burdens on the environment compared to RFs.

The emitted kg CO ₂ eq. in IFs is 20 times that of irrigated Spanish farms ( Lechon et al ., 2005 ). It is notable since our energy flow analysis showed that higher energy was consumed and higher yield was obtained in irrigated barley farms, although energy efficiency indices were not significantly different between IFs and RFs. The result of LCA justify that if lower yield in RFs is not a critical point and the country can preserve self-sufficiency in barley production, RF barley production is more beneficial from both energy and environment point of view. Furthermore, selection of the proper cultivars is an effective way to reduce environmental impacts of barley ( Niero et al. , 2015 ).

The LCA of alternative scenarios for irrigated barley farms

In Fig. 3, the three alternative scenarios were compared to conventional irrigated barley production as reference scenario (S1). All alternatives revealed some reduction of the burdens on the environment. Employing conservation tillage (S3) had the lowest effect on the impacts mitigation. However, it should be considered that Fig. 3 shows the effect of farm operations together with other activities, while these effects were high when farm operations were considered in a separate LCA model. Scenario S3 (using no tillage machinery) revealed some reductions in all environmental impacts compared to S1. A similar study on wheat ( Lares-Orozco et al ., 2016 ) showed that conventional system contributed to 13% of GWP while no tillage systems contributed to 1% of GWP. In the S3 (using biomass at power plant for electricity generation) and S4 (combination of S2 and S3), environmental impacts were significantly reduced in all categories (50-65%) especially for AD, HT, FAET, MAET and PhO by a considerable amount. Although the reduction of AC and TE were negligible, other categories had some mitigation (20-30%).

The result of S2 confirmed that remained barley straw, as a renewable source of energy, can be successfully used for cleaner energy generation and reduction of environmental impacts. The availability of agricultural waste for bioenergy production and various options in Iran is strongly recommended in different studies ( e.g . Najafi et al. , 2009). Scenario S4, which is a combination of S2 and S3 ( i.e . employing conservation tillage system in farms and using biomass for electricity generation at power plants) can be a promising approach to mitigate environmental impacts of irrigated barley production. As discussed earlier, both the yield and energy consumption of irrigated barley were twice that of rain-fed barley. While LCA analysis revealed that irrigated barley production had more environmental impacts than rain-fed barley. Table 4 shows that most of the environmental impacts from RFs were very close to scenario S4. For instance, the emitting levels of kg SO ₂ eq. as a value of acidification was 0.011 in RFs and 0.012 in S4. Furthermore, impact of S4 on the OD was much less than RFs. The consumption from natural resources intensifies the depletion of abiotic resources. S4 had around 60% fewer burden on the AD compared to S1 and S3, since biomass was used in this scenario instead of natural gas. These results are promising since clarify that when S4 is applied, the country can have suitable yield from irrigated barley farms while burdens on the environment is as low as RFs. Nevertheless, increasing yield efficiency is an effective means to reduce environmental impacts. A study in Finland ( Virtanen et al ., 2007 ) showed that N and P run-offs would decrease by ~ 5.8% and ~ 2.4%, respectively, for a 100 kg/ha barley yield increase.

In summary, the patterns of energy use for irrigated and rain-fed barley and cradle-to-farm gate LCA of both were investigated in this study. Fertilizer, diesel fuel and water for irrigation were the main contributors of energy input in irrigated farms. It is recommended that the use of fertilizer applications, green crops and suitable alternative conservation tillage systems are possible ways to reduce energy inputs. Applying efficient water pumps and farm machines are essential to mitigate both energy consumption and impacts on the environment. Literature reviews showed that other beneficial methods such as data envelopment analysis (DEA) and evolutionary algorithms can be used to further investigate the reasons of energy use inefficiency in barley farms ( Houshyar et al. , 2012 ). In the current study water obtained from wells was considered in the analyses of IFs. More correct evaluations of water use productivity in IFs can be obtained if water from rainfall is also included in the calculations.

The result of the LCA demonstrated that rain-fed barley production is environmentally more suitable than the irrigated barley. However, lower yield and net-energy gain are critical points of this system. More investigation is necessary to clarify the amount of N fertilizers leached into groundwater in the form of nitrate in barley fields. Alternative scenarios revealed that using conservation tillage systems in farm operations and planting, and biomass utilization for electricity generation can significantly mitigate environmental impacts of IFs. Accordingly, the benefits of irrigated barley production such as higher yields can be maintained for a country’s self-sufficiency simultaneously with much lower burden on the environment. Biomass can be used in some other ways; i.e . as livestock feed, in mushroom production as a soft bed etc. which would change the environmental effects of biomass that are removed from fields. The investigation on these subjects is recommended for future studies.